The present invention generally relates to an electrical filter, and more particularly, to a high frequency band-pass filter of the distributed constant type, to be used particularly as a filter of a transmitting antenna.
Commonly, a transmitter is provided with an antenna filter for suppression of unnecessary frequencies of radiation.
In the block diagram of FIG. 11 showing a general arrangement of a conventional antenna filter as referred to above, an output stage of the transmitter XR has an antenna filter FL connected to it. An antenna AN is coupled to the antenna filter FL as illustrated. For the antenna filter FL, a high frequency band-pass filter of the distributed constant type, employing dielectric coaxial resonators or strip line resonators, may be used. Normally, such a high frequency band-pass filter FL includes a plurality of resonators, e.g. four resonators V, each of which has an electrical length, for example, of .lambda./2 as in the arrangement of FIG. 11.
An antenna filter of the distributed constant type, as described above, has an insertion loss. Thus, a case where, for example, a high frequency signal of 5 W is applied to an antenna filter which has an insertion loss of 3 dB, the power delivered to the antenna will be 2.5 W, and it folows that a difference of power of 2.5 W between the input signal power and the antenna power has been consumed in the antenna filter. Thus, the efficiency is very low as observed for the transmitter as a whole.
Particularly, since a filter employing strip line resonators has resonators with low filter sharpness (Q) of the (in the range of approximately several tens to several hundreds in the microwave region), it has a high insertion loss, and if used as an antenna filter, reduces the efficiency of the transmitter as a whole to a great extent.
To overcome the disadvantage, the filter sharpness Q may be increased, to reduce the insertion loss, but generally, in filters employing dielectric coaxial resonators or strip line resonators, the size of the filter configuration and the filter sharpness Q are directly related to each other, and thus, the filter size is undesirably increased, if the filter sharpness Q is to be improved.
Accordingly, in conventional high frequency equipment such as a transmitter and the like, the problem has been that, if it is intended to reduce the loss of power in the antenna filter, the size of the entire filter is increased, while on the contrary, when it is attempted to reduce the overall size of the filter, the power loss in the antenna filter is undesirably increased, thereby presenting a bottleneck in the reduction of size of high frequency equipment.